Single chamber solid oxide fuel cell architecture for high temperature operation

ABSTRACT

The present invention provides a solid oxide fuel cell system comprising at least one fuel cell and at least one gas flow channel to deliver a reactant mixture. The fuel cell comprises at least one chamber to house at least one anode, at least one cathode, and at least one electrolyte, and the fuel cell is adapted to receive a reactant mixture comprising reactants mixed prior to delivery to the fuel cell. The one or more gas flow channels for delivering the reactant mixture have characteristic dimensions that are less than a quench distance of the reactant mixture at an operating temperature within the solid oxide fuel cell system.

FIELD OF THE INVENTION

The present invention is directed to the field of fuel cells and moreparticularly to a single chamber solid oxide fuel cell.

BACKGROUND OF THE INVENTION

Solid oxide fuel cells (SOFCs) are designed to introduce reactant gasesto two electrodes (i.e. an anode and a cathode) which are then broughtinto electrical contact via an electrolyte. Traditionally, the reactantgases are mixed within the fuel cell, with the oxidant, e.g. air, beingfirst introduced at the cathodic portion of the fuel cell, and the fuel,e.g. hydrogen or hydrocarbon, being first introduced at the anodicportion. An external load is connected to the anode and cathode causingoxygen at the cathode to react with incoming electrons from the externalcircuit to form oxygen ions. The oxygen ions migrate to the anodethrough the electrolyte and oxidize the fuel at the anode, resulting inthe liberation of electrons to the external circuit and causing acurrent flow that returns electrons to the cathode.

One development in SOFCs has been the use of a single chamber design tosimplify the cell fabrication and subsequent system operation. Thesingle chamber design requires the reactants to be mixed prior todelivery to the anode and cathode of the fuel cell. (See, for example,U.S. Pat. No. 4,248,941 to Louis et al.). Since SOFCs generally operateat or above 500° C., the potential for uncontrolled exothermic reactionsis present in any single chamber design. Uncontrolled reactions cancause the consumption of the reactants before and during the delivery ofthe reactants to the operating cell resulting in reduced efficiency orpotentially damaging explosions.

SUMMARY OF THE INVENTION

The present invention provides a solid oxide fuel cell system comprisingat least one fuel cell and at least one gas flow channel to deliver areactant mixture. The fuel cell comprises at least one chamber to houseleast one anode, at least one cathode, and at least one electrolyte, andthe fuel cell is adapted to receive a reactant mixture comprisingreactants mixed prior to delivery to the fuel cell. The one or more gasflow channels for delivering the reactant mixture have characteristicdimensions that are less than a quench distance of the reactant mixtureat an operating temperature within the solid oxide fuel cell system.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described with reference to the several figures of thedrawing, in which:

FIG. 1 shows a fuel cell system according to one embodiment of theinvention;

FIG. 2 is a cross-sectional view of the fuel cell system according toone embodiment of the invention;

FIG. 3 is a cross-sectional view of a gas flow passage with a lowcatalytic activity surface according to one embodiment of the invention;

FIG. 4 shows a fuel cell stack according to one embodiment of theinvention; and

FIG. 5 shows a fuel cell system having a heat exchanger at the inletaccording to one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention utilizes a single chamber design in a solid oxidefuel cell to simplify the cell fabrication and subsequent systemoperation. The single chamber design requires the reactants, normally agaseous fuel and air, to be mixed prior to delivery to the anode andcathode of the fuel cell. Note that in single chamber SOFCs, the anodeand cathode may be either on the same side or opposite sides of theelectrolyte. Having both on the same side facilitates the fabrication ofthe cell if photolithography and other techniques common to theelectronics industry are used for construction. Regardless of theconfiguration of the cathode and anode, the reactants are mixed prior todelivery to the fuel cell.

Since SOFCs generally operate at or above 500° C. (and potentially ashigh as 1000° C.), the potential for uncontrolled reactions is presentin any single chamber design. The present invention describes anapparatus and method for reducing the chances of uncontrolled exothermicreactions from consuming the reactants before and during the delivery ofthe reactants to the operating cell. This is accomplished by reducingthe characteristic dimensions of all gas flow passages up to a fuel cellor fuel cell stack, as well as the duct work within the fuel cell stackitself, to a size less than the quench distance of the reactant mixtureat the temperature found within the SOFC system. In general, thecharacteristic dimension is the smallest dimension that defines a flowchannel. The characteristic dimension of a flow channel having acircular cross-section is the diameter, while for a non-circular flowchannel, including a flow channel having an aspect ratio approachingthat of parallel plates, the characteristic dimension is the height ofthe channel. Careful design of the flow channels must take place at alllocations where fuel and air exist combined into one reactant mixture

In combustion terms, the quench distance is the distance below which aflame will not propagate. For example, in the case of a tube having adiameter below the characteristic dimension (quench diameter), a flameis quenched by the tube wall and cannot flash back. Quench distance isdetermined in part by the fuel/oxidant mixture, the operationaltemperature, and the operational pressure. The quench distance isgenerally smaller for more reactive fuels, decreases as temperatureincreases, and increases as pressure decreases. For a more detailedanalysis of quench distance calculation, see Kuo, Kenneth K., Principlesof Combustion, John Wiley, New York, 1986, pp. 326-329.

Referring now to the figures of the drawing, the figures constitute apart of this specification and illustrate exemplary embodiments to theinvention. It is to be understood that in some instances various aspectsof the invention may be shown exaggerated or enlarged to facilitate anunderstanding of the invention.

FIG. 1 illustrates a fuel cell system 10 according to one embodiment ofthe invention. A fuel cell 20 comprises an anode 14 and cathode 16electrically connected via an electrolyte 18 and positioned on a supportstructure 12. Reactants, fuel and oxidant, are mixed prior to deliveryto the fuel cell 20. The reactant mixture 50 is delivered to theoperating fuel cell 20 via at least one gas flow passage 40. The gasflow passage 40 has a characteristic dimension (shown as the insidediameter 42) that is less than the quench distance of the reactantmixture at the temperature found within the SOFC system. Because thediameter 42 of the gas flow passage 40 is less than the quench diameter,a flame cannot propagate along the gas flow passage, and the chance ofuncontrolled reactions or explosions is reduced or even eliminated. Thefuel cell is housed in a single chamber 30 providing a open space abovethe fuel cell that allows the reactant mixture 50 to contact the fuelcell. As shown, the chamber is formed as part of a delivery structure 32which is attached in a leak-tight manner to support structure 12. Thechamber 30 should also have characteristic dimensions that are less thanthe quench distance of the reactant mixture at the temperature foundwithin the SOFC system. As shown, the chamber 30 has a characteristicdimension height 42 that corresponds with the diameter dimension 42 ofthe gas flow passage 40 and which is less than the quench distance ofthe reactant mixture.

Contacts 22 comprising an electrically conductive material are attachedto the anode and cathode in finger like formations and provide contactpoints at the edge of the support structure 12. A completed circuit iscreated by connecting an eternal load 24. Spent reactant mixture andother byproducts such as water vapor can be expelled out of exhaust port26.

For hydrocarbon/air mixtures at temperatures typical of SOFC operation(in the range of approximately 350° C. to 750° C.), the largestcharacteristic dimension of any flow path carrying the mixed reactantsshould not exceed 1 mm, and preferably should be less than 500micrometers. For hydrogen/air mixtures, the largest characteristicdimension of any flow path carrying the mixed reactants should notexceed 0.5 mm, and preferably should be less than 100 micrometers. Notethat large excess air flow rates will help to reduce the likelihood ofreactions. There exists a lower flammability limit, at room temperature,under which the air/fuel mixture will not react. However, as thereactant mixture temperature increases, the benefits of excess air flowrates is diminished. Hence, a combination of both small passages andexcess air flow rates is desirable in the design of the single chamberSOFC systems.

FIG. 2 shows a cross-sectional view of the fuel cell system according toone embodiment of the invention. The point of view is shown in FIG. 1.The fuel cell 20 comprised of the anode, cathode and electrolyte ishoused in a single chamber 30. Reactant mixture is supplied through agas flow passage 40 having a characteristic dimension 42 less than thequench distance of the reactant mixture at the operating temperature ofthe SOFC system. The single chamber 30 also has a characteristicdimension less than this quench distance.

The present invention further discloses the use of low catalyticactivity surfaces for all delivery channels in contact with the elevatedtemperature mixed reactants. Oxidized stainless steel, quartz, andalumina are examples of low catalytic activity materials known in thecombustion literature not to be active at promoting reactions. The useof low catalytic activity surfaces on the gas flow channels exposed tothe reactant mixture help limit uncontrolled reactions. FIG. 3illustrates the use of a low catalytic activity surface 44 as applied toa gas flow passage 40. The quench diameter of the passage shown ismeasured from the inside of the passage (including the low catalyticactivity surface); however, the width of the surface has been greatlyexaggerated for purposes of clarity and in reality would be very smallcompared to the overall size of the passage. Low catalytic activitysurfaces may be applied to all gas flow channels that are exposed to thereactant mixture including gas flow passages and ducts.

FIG. 4 illustrates a fuel cell stack 100 according to one embodiment ofthe invention. The fuel cell stack 100 comprises fuel cells 101-105electrically connected in series on support structure 112, a deliverystructure 132 defining delivery means and one or more chambers 130 tohouse fuel cells, and a top structure 134 to help prevent leaks in thefuel cell stack, all of which are attached together to form the fuelcell stack architecture. Gas flow passages 40 deliver the reactantmixture 50 to the fuel cell stack. Gas flow ducts 140 machined into thebottom of the delivery structure 132 then deliver the reactant mixture50 within the fuel cell stack to the one or more chambers 130 and placethe reactant mixture in contact with the fuel cells. For the aspectratios of the chambers and ducts shown, the characteristic dimensiondefining the flow path is the height. Like the gas flow passages 40, thegas flow ducts 140 have characteristic dimensions (shown as height 142)that are less than the quench distance of the reactant mixture at thetemperature found within the SOFC system. The one or more chambers 130housing the fuel cells should also have characteristic dimensions lessthan this quench distance.

An exhaust mechanism, including exhaust ports 126 and exhaust channel136, removes spent reactant mixture and byproducts such as water vapor.Fuel cell stacks may be further connected in parallel or series viacontacts 122 to produce any desired power output for an external load24.

For hydrocarbon/air mixtures, the largest characteristic dimension 142of any flow path in the gas flow ducts 140 carrying the mixed reactantsshould not exceed 1 mm, and preferably should be less than 500micrometers at temperatures characteristic of SOFC operation (in theapproximate range of 350° C. to 750° C.). For hydrogen/air mixtures, thelargest characteristic dimension 142 of any flow path in the gas flowducts 140 carrying the mixed reactants should not exceed 0.5 mm, andpreferably should be less than 100 micrometers.

In another embodiment, gas flow channels and housing chambers couldpotentially be designed so that both height and width of thenon-circular channels and chambers have dimensions less than a quenchdistance of the reactant mixture at an operating temperature within theSOFC system. This is most applicable where the height and width ofnon-circular channels and chambers are approximately equal.

FIG. 5 shows a fuel cell system 10 having an heat exchanger 60incorporated into the system. Fuel and oxidant can be mixed at the inlet62 of the heat exchanger to produce the reactant mixture. Careful designof the flow channels must take place at all locations where fuel and airexist combined into one reactant mixture, including the reactant mixturetransportation means within the heat exchanger. The incorporation of aheat exchanger into the fuel cell system is equally applicable to a fuelcell stack.

Other embodiments of the invention will be apparent to those skilled inthe art from a consideration of the specification or practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with the true scope and spiritof the invention being indicated by the following claims.

1-14. (canceled)
 15. A method of operating a solid oxide fuel cellsystem, comprising: delivering a reactant mixture to said at least onefuel cell through at least one gas flow channel, wherein said at leastone gas flow channel has a characteristic dimension less than a quenchdistance of said reactant mixture at an operating temperature of thefuel cell.
 16. The method of claim 15 further comprising the step of:forming said reactant mixture by mixing reactants prior to delivery tosaid at lest one fuel cell.
 17. The method of claim 15 furthercomprising the step of: delivering said reactant mixture through atleast one gas flow duct within a fuel cell stack formed by at least twofuel cells, wherein said at least one gas flow duct has a characteristicdimension less than the quench distance of said reactant mixture at theoperating temperature of the fuel cell.
 18. The method of claim 15further comprising the step of: delivering said reactant mixture to aheat exchanger prior to delivery of the reactant mixture to said atleast one fuel cell.
 19. The method of claim 15 further comprising thestep of providing an excess air flow rate during said delivery step. 20.The method of claim 15 further comprising the step of applying at leastone low catalytic activity layer on a surface of said at least one gasflow channel where exposed to said reactant mixture.
 21. A method ofmaking a solid oxide fuel cell system, comprising: manufacturing all gasflow channels and chambers exposed to a reactant mixture within thesolid oxide fuel call system to have characteristic dimensions less thana quench distance of said reactant mixture at a temperature found withinthe solid oxide fuel cell system.
 22. The method of claim 21 whereinsaid gas flow channels and chambers comprise: at least one gas flowpassage, at least one gas flow duct, and at least one housing chamber.23-25. (canceled)